U.S. patent number 7,511,511 [Application Number 11/754,683] was granted by the patent office on 2009-03-31 for specific absorption rate measuring system, and a method thereof.
This patent grant is currently assigned to Nippon Telegraph and Telephone Corporation, NTT DoCoMo, Inc.. Invention is credited to Tadao Nagatsuma, Teruo Onishi, Naofumi Shimizu, Hiroyoshi Togo, Shinji Uebayashi, Ryo Yamaguchi.
United States Patent |
7,511,511 |
Onishi , et al. |
March 31, 2009 |
Specific absorption rate measuring system, and a method thereof
Abstract
A biological tissue equivalent phantom unit to be used by the
specific absorption rate measuring system for evaluating absorption
of electromagnetic wave energy includes a biological tissue
equivalent phantom for absorbing an electromagnetic wave. In
addition, two or more electro-optical crystals are arranged at two
or more measurement points in the biological tissue equivalent
phantom. The electro-optical crystals have a dielectric constant
that is approximately equal to that of the biological tissue
equivalent phantom. Two or more optical fibers are laid in the
biological tissue equivalent phantom for optically connecting each
of the electro-optical crystals to an external destination.
Inventors: |
Onishi; Teruo (Yokohama,
JP), Yamaguchi; Ryo (Zushi, JP), Uebayashi;
Shinji (Yokohama, JP), Nagatsuma; Tadao (Atsugi,
JP), Shimizu; Naofumi (Atsugi, JP), Togo;
Hiroyoshi (Atsugi, JP) |
Assignee: |
NTT DoCoMo, Inc. (Tokyo,
JP)
Nippon Telegraph and Telephone Corporation (Tokyo,
JP)
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Family
ID: |
35781584 |
Appl.
No.: |
11/754,683 |
Filed: |
May 29, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070236229 A1 |
Oct 11, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11263946 |
Nov 2, 2005 |
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Foreign Application Priority Data
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Nov 2, 2004 [JP] |
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2004-319387 |
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Current U.S.
Class: |
324/632; 324/631;
343/703; 324/247 |
Current CPC
Class: |
G01R
29/0885 (20130101) |
Current International
Class: |
G01N
22/00 (20060101); G01R 29/08 (20060101) |
Field of
Search: |
;324/632,631 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2004-12468 |
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Jan 2004 |
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JP |
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WO 01/86311 |
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Nov 2001 |
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WO |
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Other References
US. Appl. No. 11/873,934, filed Oct. 17, 2007, Kiminami et al.
cited by other .
M. Berwick, et al., "High Precision Electromagnetic Field Sensors
For Monitoring Communications Devices", Singapore ICCS, Conference
Proceedings, XP-010150094, vol. 3, Nov. 14, 1994, pp. 944-948.
cited by other .
S. Wakana, et al., "Novel Electromagnetic Field Probe Using
Electro/Magneto-Optical Crystals Mounted on Optical-Fiber Facets
For Microwave Circuit Diagnosis", 2000 IEEE MTT-S International
Microwave Symposium Digest, XP-000967537, vol. 3 of 3, Jun. 11,
2000, pp. 1615-1618. cited by other .
Volker Hombach, et al., "The Dependence of EM Energy Absorption
Upon Human Head Modeling at 900 MHz", IEEE Transactions on
Microwave Theory and Techniques, IEEE Service Center, XP-002125147,
vol. 44, No. 10, Oct. 1996, pp. 1865-1873. cited by other .
A Cruden et al., Current and Voltage Measurement Using Optical
Crystal Based Devices, Nov. 2, 1994, Low Frequency Power
Measurement and Analysis (Digest No. 1994/203), IEE Colloguium, pp.
8/1-8/5. cited by other.
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Primary Examiner: Gutierrez; Diego
Assistant Examiner: Zhu; John
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of U.S. patent application Ser.
No. 11/263,946, filed Nov. 2, 2005, and claims priority to Japanese
Patent Application No. 2004-319387, filed Nov. 2, 2004, the entire
contents of these applications are incorporated herein by
reference.
Claims
What is claimed is:
1. A specific absorption rate measuring system for evaluating
absorption of electromagnetic wave energy, comprising: a biological
tissue equivalent phantom for absorbing electromagnetic wave
energy; two or more electro-optical crystals that are arranged at
two or more measurement points in the biological tissue equivalent
phantom, the electro-optical crystals having a dielectric constant
approximately equal to the dielectric constant of the biological
tissue equivalent phantom; two or more optical fibers laid in the
biological tissue equivalent phantom for coupling each of the
electro-optical crystals to an external destination; and a specific
absorption rate measurement unit configured to determine a specific
absorption rate of the biological tissue equivalent phantom, based
on the response of the electro-optical crystals conveyed thereto
via the two or more optical fibers wherein the specific absorption
rate measurement unit derives a specific absorption rate (SAR) of
the biological tissue equivalent phantom, the SAR being defined by
an equation that is a function of a conductivity .sigma. of the
biological tissue equivalent phantom, a density .rho. of the
biological tissue equivalent phantom, a constant K determined by
the crystallographic axis of the electro-optical crystals, and a
phase difference .GAMMA..
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a biological tissue equivalent
phantom unit (phantom unit) used by a specific absorption rate
measuring system for evaluating absorption of electromagnetic wave
energy; a specific absorption rate measuring system using the
phantom unit; and a method thereof.
2. Description of the Related Art
In recent years and continuing, requirements are increasing for
quantitatively evaluating a thermal effect caused by an
electromagnetic wave emitted by a wireless radio transmitter; and a
system that is capable of accurately and swiftly measuring a
specific absorption rate (SAR), which is an index of the reaction
of the electromagnetic wave on a living body, such as a human, is
desired.
A SAR value is proportional to an electric field (|E|.sup.2), and
is often used for evaluating the energy absorbed by a human body
when a cellular phone is used near the human body, SAR being
defined by the following Equation 1. SAR=.sigma.|E|.sup.2/.rho.
[Equation 1]
Here, .sigma. and .rho. represent conductivity [S/m] and density
[kg/m3], respectively, of the biological tissue equivalent
phantom.
Usually, when measuring SAR, an electric-field measuring method is
used, wherein a short dipole detects an electric field generated in
a medium (live body), which electric field is converted into SAR
using Equation 1.
FIG. 1 shows a conventional specific absorption rate measuring
system 100 that includes a simulated body (phantom) 101 that
simulates an electric constant of a human body with liquid, a
container 102 into which the liquid is provided, a probe 103 for
detecting an electric field, a probe scanner 104, a signal cable
105, an electric field detection apparatus 106, and a processor
apparatus 107 for measurement operations and data analysis.
Here, the electric field generated in the phantom is measured by
arranging a measuring target instrument 108, such as a cellular
phone, near the specific absorption rate measuring system 100 as
shown in FIG. 1. The probe 103 for detecting the electric field is
scanned in three dimensions by the probe scanner 104, and SAR is
measured.
FIG. 2 shows another specific absorption rate measuring system 200
that includes a phantom 121 that simulates the electric constant of
the human body with a solid-state object, a probe 122 for detecting
the electric field, a cable 123 for signal transmissions, an
electric-field detection apparatus 124, a processor apparatus 125
for measurement operations and data analysis, and a scanner
126.
The electric field generated in the phantom is measured by
arranging a measuring target instrument 127, such as a cellular
phone, near the specific absorption rate measuring system 200 as
shown in FIG. 2. However, unlike the conventional example shown by
FIG. 1, the cellular phone 127 is moved by the scanner 126, and SAR
is measured.
In either of the conventional examples, the probe 103 or 122, as
applicable, for detecting the electric field is required. Each of
the probes 103 and 122 for detecting the electric field includes a
detecting element 110 as shown in detail on the right-hand side of
FIG. 1. There, an electric field is detected by short dipole
elements 111 and 112. Then, the electric field is detected by a
Schottky diode 113 inserted in a gap, and a detected result in the
form of an electrical signal is provided to the corresponding
electric-field detection apparatuses 106 and 124 through high
resistance wires 114. That is, the Schottky diode 113 detects a
voltage generated by the short dipole elements 111, 112 formed with
conductors, the length of which is about 2 to 5 mm.
However, since the short dipole antenna and the high resistance
wires, both being conductive, are present in the electric field to
be measured, the electromagnetic field distribution near the
detecting element 110 is disturbed. This is a problem of the
electric-field measuring method. Further, since it is difficult to
reduce the length of the dipole elements 111, 112, it is expected
that the disturbance will become greater as the frequency becomes
higher.
Then, in an attempt to reduce the disturbance to the
electromagnetic field generated by the measuring target 108, 127
(e.g. a cellular phone), the disturbance being due to the probe
104, 122 for detecting the electric field, an electric-field sensor
300 using an optical waveguide type modulator and a laser beam has
been developed as shown in FIG. 3.
The electric-field sensor 300 includes a laser luminous source 131,
an electric-field probe 132, an optical waveguide type modulator
133, a minute dipole 134 that consists of metal, and an optical
receiving unit 135.
Since the electric-field sensor 300 is configured only by
dielectric materials, except for the minute dipole 134, it is
capable of measuring the electric field with a precision that is
higher than the electric-field detecting methods that use the high
resistance wires.
Nevertheless, since the short dipole is used according to the
electric-field measuring method using the electric-field sensor
300, wherein the optical waveguide type modulator and the laser
beam are used, the disturbance remains, although the disturbance
becomes smaller than in the case of the electric-field measuring
methods using the high resistance wires. Further, since the probe
for detecting the electric field, or a 3-dimensional electric-field
sensor, is moved in the liquid phantom for measuring SAR, the
liquid (a phantom solvent) is agitated, and noise is generated by
vibration of the probe or sensor. If a time until the solvent
settles into a steady state is waited for in order to avoid the
noise, measurement will take a long time. If two or more
electric-field sensors are arranged in two dimensions or three
dimensions in the phantom in order to shorten the measuring time,
the aggregate of the sensors (short dipoles) will behave as a
conductor, and will generate a great disturbance to the
electromagnetic field to be measured. Consequently, a SAR
distribution that is different from actual may be measured, which
is a problem.
SUMMARY OF THE INVENTION
The present invention provides a specific absorption rate measuring
system, a biological tissue equivalent phantom unit, and a method
thereof that substantially obviate one or more of the problems
caused by the limitations and disadvantages of the related art.
Features of the present invention are set forth in the description
that follows, and in part will become apparent from the description
and the accompanying drawings, or may be learned by practice of the
invention according to the teachings provided in the description.
Problem solutions provided by the present invention will be
realized and attained by a specific absorption rate measuring
system, a biological tissue equivalent phantom unit, and a method
thereof particularly pointed out in the specification in such full,
clear, concise, and exact terms as to enable a person having
ordinary skill in the art to practice the invention.
To achieve these solutions and in accordance with the purpose of
the invention, as embodied and broadly described herein, the
invention provides a specific absorption rate measuring system, a
biological tissue equivalent phantom unit, and a method thereof as
follows.
An aspect of the present invention provides a biological tissue
equivalent phantom unit that is to be used by a specific absorption
rate measuring system for evaluating absorption of electromagnetic
wave energy. The biological tissue equivalent phantom unit includes
a biological tissue equivalent phantom for absorbing an
electromagnetic wave, two or more electro-optical crystals that
have a dielectric constant approximately equal to the dielectric
constant of the biological tissue equivalent phantom, the
electro-optical crystals being arranged at two or more measurement
points set up in the biological tissue equivalent phantom, and two
or more optical fibers provided in the biological tissue equivalent
phantom for connecting each of the electro-optical crystals to an
external destination.
According to another aspect of the present invention, a high
dielectric constant material is applied to the surface of the
optical fibers of the biological tissue equivalent phantom
unit.
Another aspect of the present invention provides a specific
absorption rate measuring system for evaluating the absorption of
the electromagnetic wave energy using the biological tissue
equivalent phantom unit. The specific absorption rate measuring
system includes a luminous source for emitting a light, a
polarization regulator for adjusting a polarization state of the
light emitted by the luminous source, an optical-path switcher for
switching the light output by the polarization regulator to each
electro-optical crystal one by one, and a specific absorption rate
measuring unit for measuring a specific absorption rate by
detecting the light reflected by the electro-optical crystal.
Another aspect of the present invention provides a specific
absorption rate measuring method of evaluating absorption of the
electromagnetic wave energy using the biological tissue equivalent
phantom that receives irradiation of the electromagnetic wave. The
specific absorption rate measuring method includes a step of
arranging two or more electro-optical crystals that have a
dielectric constant approximately equal to that of the biological
tissue equivalent phantom to two or more measuring points in the
biological tissue equivalent phantom, a step of sequentially
providing the light to each of the electro-optical crystals through
the optical-path switcher, the light being irradiated by the
luminous source, where the polarization state of the light is
adjusted, a step of reflecting the light that is provided to the
electro-optical crystals, a step of leading the light reflected
from the electro-optical crystal to an analyzer, and a step of
converting the light that passes the analyzer into an electrical
signal by a photodetector, and obtaining the specific absorption
rate.
According to another aspect of the present invention, the step of
reflecting the light that is provided to the electro-optical
crystals of the specific absorption rate measuring method is a step
of reflecting the light by a dielectric reflective film prepared on
a surface countering a surface, through which the light is
provided, of the electro-optical crystal.
According to another aspect of the present invention, the step of
sequentially providing the light to each of the electro-optical
crystals through the optical-path switcher of the specific
absorption rate measuring method is a step of sequentially
providing the light to each of the electro-optical crystals by
selecting an optical fiber by the optical-path switcher, wherein
the optical-path switcher is connected to each of the
electro-optical crystals.
According to another aspect of the present invention, as for the
specific absorption rate measuring method, a high dielectric
constant material is applied to the surface of the optical fiber
such that the equivalent dielectric constant of the optical fiber
becomes substantially equal to the dielectric constant of the
biological tissue equivalent phantom.
As described above, according to the present invention, since the
electric-field detecting element is constituted by nonmetallic
materials, it is possible to measure the SAR distribution without
the disturbance that is generated in the case of the conventional
technology. Further, since the electro-optical crystals having a
dielectric constant approximately equal to that of the phantom are
used as a sensor head, reflection due to difference of the
dielectric constants is reduced, and the SAR distribution can be
more accurately measured. Furthermore, since spatial resolution of
the measurement is proportional to a diameter of a beam of the
light that penetrates the electro-optical crystal, the spatial
resolution can be raised, theoretically, to as small as the
wavelength of the light (several .mu.m). Furthermore, since a
change in a refractive index of the electro-optical crystal at the
measuring point depends on deviation of a dipole that follows the
electromagnetic wave, the SAR measurement is available in a wide
band range, from the MHz band to the THz band.
According to the present invention, disturbance of the electric
field in the electro-optical crystal by interface reflection is
reduced, and the influence of the interface reflection on the
electromagnetic field near the electro-optical crystals is reduced
by using the electro-optical crystal that has a dielectric constant
approximately equal to that of the phantom. Therefore, the specific
absorption rate measuring system capable of obtaining an accurate
SAR distribution is realized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a conventional specific absorption
rate measuring system 100;
FIG. 2 a schematic diagram of another conventional specific
absorption rate measuring system 200;
FIG. 3 is a schematic diagram of another conventional specific
absorption rate measuring system 300 using an optical waveguide
type modulator, a laser beam, and an electric-field sensor;
FIG. 4 is a block diagram of a specific absorption rate (SAR)
measuring system according to an embodiment of the present
invention;
FIG. 5 is a perspective diagram showing a phantom according to the
embodiment of the present invention; and
FIG. 6 gives a graph showing an error of field strength in an
electro-optical crystal, the error being due to difference in
dielectric constants.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, embodiments of the present invention are
described with reference to the accompanying drawings, wherein
common reference numbers are assigned to items that have the same
functions, and the descriptions are not repeated.
FIG. 4 is a block diagram of a specific absorption ratio measuring
system 40 according to the embodiment of the present invention. The
specific absorption rate measuring system 40 includes a biological
tissue equivalent phantom unit 42 that is constituted by a
simulated human body (phantom) 1 consisting of liquid, gel, a
solid-state object, etc., for simulating the electric constant of
the human body, electro-optical crystals 3 that have a dielectric
constant approximately equal to that of the phantom 1, and bare
fibers 10.
The specific absorption rate measuring system 40 further includes
an electromagnetic wave generator 2 such as a cellular phone, a
linearly polarized luminous source 4 such as DFB-LD, a polarization
maintenance fiber (PMF) 5, a circulator 6, a polarization regulator
7 that includes a 1/4 wave plate and a 1/2 wave plate, single mode
fibers (SMF) 8, an optical-path switcher 9 that is formed by MEMS
technology and PLC technology, and a specific absorption rate
measuring unit 44.
The specific absorption rate measuring unit 44 includes an analyzer
11, a photodetector 12, an electrical signal line 13, a signal
processing unit 14, and a SAR distribution image display 15.
The specific absorption rate measuring system 40 is for measuring
an electric field in the phantom 1 using the electro-optical
crystals 3, the electric field being generated by the
electromagnetic wave generator 2 (such as a cellular phone)
arranged near the phantom 1, as shown in FIG. 4.
The linearly polarized light irradiated by the luminous source 4 is
provided to the polarization regulator 7 via the circulator 6 and
the polarization maintenance fiber (PMF) 5. The polarization
regulator 7 changes the polarization of the linearly polarized
light into a predetermined polarization state, and outputs the
light.
The polarization state is determined by a crystallographic axis of
the electro-optical crystal 3 arranged in the phantom 1 and a
vibrating direction of the electric field generated by the
electromagnetic wave generator 2. For example, when detecting an
electric field that vibrates in parallel to the y-axis using CdTe,
which is a lead marcasite type crystal, crystallographic faces
(001), (100), and (010) of CdTe are perpendicularly arranged to the
y, x, and the z axes, respectively; or to the y, z, and x axes,
respectively; and the polarization regulator 7 is adjusted so that
the polarization axis of the linear polarization or an elliptical
polarization may become parallel to the x axis or the z axis.
The light, the polarization of which has been adjusted, is
transmitted via the single mode fiber (SMF) 8 to the optical-path
switcher 9, then to the electro-optical crystals 3.
The light is reflected by a dielectric reflective film prepared on
a surface of the electro-optical crystal 3, the surface countering
a surface through which the light is provided, and the light goes
back along the incidence path. When the light goes back along the
incidence path within the electro-optical crystal 3, a phase
difference arises between components of the predetermined
polarization due to a refractive-index change (Pockels effect) that
is proportional to the field strength to the first power that is
applied, and the polarization state is changed, i.e., polarization
modulation occurs.
When CdTe is arranged, e.g., as described above, a phase difference
.GAMMA. arises between polarization components that are parallel to
the x axis and the z axis, the phase difference F being expressed
by the following Equation 2. .GAMMA.=(2
.pi./.lamda.)n.sub.0.sup.3r.sub.41Ed [Equation 2]
Here, .lamda., n.sub.0, r.sub.41, E, and d represent the wavelength
[m] of the incident light, the refractive index of the
electro-optical crystal 3, the Pockels constant [m/V], field
strength [V/m], and the length [m] of the electro-optical crystal 3
in a direction of the oscillation of the electric field,
respectively.
The light that is reflected and polarization-modulated reaches the
circulator 6 through the optical-path switcher 9 and the
polarization regulator 7; then, the light is branched to the
analyzer 11 by the circulator 6. The modulation component of the
branched light is taken out by the analyzer 11, and is converted
into an electrical signal by the photodetector 12. The amplitude of
the electrical signal is proportional to the field strength of the
electromagnetic wave that is measured. The amplitude of the
electrical signal is converted into a SAR value by the signal
processing unit 14. Such values, with position information
attached, constitute a SAR distribution that can be displayed by
the SAR distribution image display 15.
According to the electric-field measuring method, the specific
absorption rate (SAR) is defined by Equation 1. According to the
present embodiment, by using the specific absorption rate measuring
system 40, SAR can be defined by the following Equation 3, based on
Equations 1 and 2. SAR=.sigma.K|.GAMMA..sup.2|/.rho. [Equation
3]
Here, K is a constant determined by the crystallographic axis of
the electro-optical crystal 3 and the vibrating direction of the
electric field irradiated by the electromagnetic wave generator 2.
The constant K can be expressed by the following Equation 4 when
CdTe is arranged as described above. K=.lamda./(2
.pi.n.sub.0.sup.3r.sub.41d) [Equation 4]
Further, when detecting the electric field that vibrates parallel
to the x (or z) axis by using CdTe, the crystallographic faces
(110), (1(1 bar) 0), and (001) of CdTe are arranged perpendicularly
to the x(z), y, and z(x) axes, respectively; then the polarization
regulator 7 is adjusted so that the polarization axis of the linear
polarization or the elliptical polarization may become parallel to
the x(z) axis. In this case, a phase difference .GAMMA. expressed
by the following Equation 5 arises between the polarized components
that incline 45.degree. to the x axis and the z axis. .GAMMA.=(2
.pi./.lamda.)ln.sub.o.sup.3r.sub.41E [Equation 5]
Here, "l" represents the length [m] of the electro-optical crystal
3 in the direction of the light passage. Further, when SAR is
defined by Equation 3, the constant K can be expressed by the
following Equation 6. K=.lamda./(2 .pi.1n.sub.0.sup.3r.sub.41)
[Equation 6]
As described above, according to the specific absorption rate
measuring system 40 of the embodiment, since the electric-field
detecting element is constituted from dielectric materials, the
disturbance to the electromagnetic field to be measured is removed,
the disturbance conventionally being due to the aggregate of the
short dipoles, and the disturbance being the problem with the
conventional electric-field measuring method. The dielectric
constants of the phantom 1 are prescribed by ARIB. Although
reflection (Fresnel reflection) of the electromagnetic wave can
arise at the interface between the phantom 1 and the
electro-optical crystal 3 depending on the kind of the
electro-optical crystal 3 according to the difference in the
dielectric constants, such reflection is very small as compared
with the disturbance due to the aggregate of short dipoles.
FIG. 6 shows the field strength in the electro-optical crystal 3 in
consideration of the reflection at the interface when there is no
absorption of the electromagnetic wave within the electro-optical
crystal 3. In calculation, a model is assumed wherein the
electromagnetic wave is perpendicularly provided to the
electro-optical crystal 3 that is semi-infinite in size, and as the
relative permittivity of the phantom, a value 40.5 at 1450 MHz that
is specified by ARIB is used. Calculation results show that a true
value can be obtained by compensating for the electric field that
is measured by about 10% for the reflection in the case of CdTe.
Further, it is considered that the influence on the measured
electromagnetic field by the reflection is proportional to an area
ratio that the electro-optical crystal 3 occupies. Given that the
minimum spatial resolution of the SAR measurement is 1 mm, and that
the minimum processing size of the electro-optical crystal 3 is
about 100 .mu.m, if the reflection factor per mm.sup.2 is converted
by the area ratio, it becomes about 1% of 1/100, which can
practically be disregarded. It is also possible to measure SAR
without compensation if electro-optical crystals that have a
dielectric constant value approximately equal to the dielectric
constant value of the phantom are used, such electro-optical
crystals including LN, LT, and KD*P. The electrical properties of
LN, LT, and KD*P and the error in the measured electric field due
to the reflection are shown in Table 1.
TABLE-US-00001 TABLE 1 Electrical properties and error in measured
electric field due to interface reflection of electro-optical
crystals Error in Electro- Pockels measured optical constant
Relative electric crystals (.times.10.sup.-12 m/V) permittivity
field LiNbO.sub.3 (LN) 19 28 0.8% LiTao.sub.3 (LT) 22 43 <0.01%
KD.sub.2PO.sub.4 (KD*P) 24.1 48 <0.2%
For the same reason, about 28% of reflection occurs at the bare
fiber 10 that connects the optical-path switcher 9 and the
electro-optical crystal 3, which reflection may cause a disturbance
to the electromagnetic field to be measured. The diameter of a
common bare fiber is 250 .mu.m including a covering layer, and the
reflection factor per mm.sup.2 of the cross section is 1/16 (about
1.8%). The covering layer is provided in consideration of a micro
bend property at low temperatures. However, since the bare fiber 10
of the specific absorption rate measuring system 40 according to
the embodiment is covered by the phantom 1, a clad fiber having a
diameter of 80 .mu.m without a covering layer can be used. By using
the clad fiber, the reflection factor per mm.sup.2 can be lowered
to 0.2% or less.
With reference to FIG. 5, if N electro-optical crystals 3 are
arranged in the direction of the y-axis, the number of the bare
fibers 10 on the optical-path switcher 9 side per mm.sup.2 is N,
and the reflection factor per mm.sup.2 becomes less than
0.2.times.N%. If the reflection factor is tolerated to be up to
10%, the number of the electro-optical crystals 3 that can be
arranged in the direction of the y-axis becomes 50. If they are
arranged at intervals of 1 mm, the length wherein the
electro-optical crystals 3 are arranged in the direction of the
y-axis is 50 mm. Since the size of the phantom that simulates the
head is about 300 mm, the reflection by the optical-path switcher 9
may become great.
On the other hand, it is possible to prevent the reflection of the
electromagnetic wave from occurring by applying a material having a
great dielectric constant to the surface of the bare fibers 10,
making the equivalent dielectric constant equal to the phantom.
Since the direction of the main axis and sintering temperature can
adjust the specific inductive capacity to a range between 40 and
120, TiO.sub.2 and BaTiO.sub.3 that have a sintering temperature
lower than a softening temperature (about 1500.degree. C.) of glass
are suitable as the material to be applied.
Further, the present invention is not limited to these embodiments,
but variations and modifications may be made without departing from
the scope of the present invention.
* * * * *